In recent years, digitization of picture information has been advanced, which leads to a rapid increase of cases where a picture signal is transmitted as a digital signal, although it has been conventionally transmitted as an analog signal.
In conventional CRT, LCD and the like, a gradation control has been made by applying an analog voltage corresponding to a desired analog gradation level to a display apparatus. Then, a variety of digital gradation control methods have come into practical use with the digitization of the picture signal. A complex DAC (Digital-Analog Converter) is not necessary in the digital gradation control method, and therefore simplification of a circuit configuration is expected as compared with a conventional analog gradation control method. Methods for gradation representation include a time divisional representation method and an area gradation representation method. The respective representation methods will be described below.
In the time divisional representation method, switching between a first gradation level and a second gradation level which a pixel represents is made temporally, a time-average is obtained as a third gradation level between the first gradation level and the second gradation level. The method is useful to realize multiple-gradation representation in a display apparatus capable only of carrying out binary representation by changing the time for representation with the first gradation level and the time representation with the second gradation level, namely, by controlling the pulse widths of them. The method is used for a PDP, a ferroelectric LCD, and some of EL represents.
As an area gradation representation method, an “Active Matrix Type Liquid Crystal Display” is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 10-68931), in which multiple-gradation representation is presented by combining a plurality of pixels, and a simple control configuration can be attained without a complicated multiple-gradation control. In the active matrix type liquid crystal display, a pixel is divided into sub-pixels, and half-tone representation is provided based on a representation area for a picture signal corresponding to binary representation. The active matrix type liquid crystal display includes a plurality of data signal lines arranged in one direction, a plurality of scanning signal lines arranged in a direction crossing the data signal lines, and a plurality of pixels provided in a matrix form. The active matrix type liquid crystal display is characterized in that a data signal line driving circuit for providing an image data for the data signal lines is composed of polysilicon thin film transistors formed on the same substrate together with the pixels, each pixel is composed of a plurality of sub-pixels, and each sub-pixel is driven in binary representation.
Also, Japanese Patent No. 2576765 discloses a “Liquid Crystal Display” with high visibility and a wide visual angle dependency. In this liquid crystal display, one pixel is composed of 2n regions which are made from a combination of n regions (n is an integer equal to or higher than 2) in which orientations of liquid crystal are different and two regions in which voltages applied to the liquid crystal are different from each other and an area ratio of a high voltage region to a low voltage region is from 4:6 to 3:7.
Also, a “Display Apparatus” using a driving method in the digital gradation representation is disclosed in Japanese Laid Open Patent Application (JP-P2000-206922A), in which the area gradation representation method and the time divisional driving method are combined, no gradation inversion due to an accumulation response occurs, and a good multiple-gradation representation is attained. In a method of driving gradation representation in which a pixel division ratio is S1:S2=1:2 and a time division ratio is T1:T2:T3=1:4:16, for example, the time division ratio is set as T1:T2:T3:T4=1:4:8:8. In this display apparatus, if there is an area error d in S2, the gradation transition is caused for one level, for example, from a level 23 to a level 24 but a gradation error is reduced by using the above-mentioned time division ratio.
Japanese Laid Open Patent Application (JP-A-Heisei 11-231827) discloses an “Image Display Apparatus” which can reduce occurrence of a false contour of dynamic picture image less than ever before. In this image display apparatus, one TV field is configured by temporally arranging N sub-fields, each of which has a brightness weight, and a picture of the TV field is presented in multiple-gradation representation by lighting desired sub-fields. The image display apparatus is characterized by including: a selecting section for selecting one gradation level according to the amount of the motion of an input picture from gradation levels that can be displayed by combining any ones of 0, W1, W2 to WN as the brightness weights of the sub-fields; and a sub-field lighting section for lighting up the sub-field which represents the selected gradation level.
Although several area gradation representation methods have been proposed ever, the fact that there are problems with regard to picture quality was mentioned by Atsushi Togami et al. in “Estimation of Shape Effect on Area Intensity Method with DT-CNN” in pages 391-398 of “Technical Report of IEICE. NC-96-206 (1997.3)”. According to the paper, deterioration of picture quality like a flaw occurs in a gray scale portion, because movement of a center of gradation is large in some portion in the area gradation representation method.
Consider a simple example. As shown in FIG. 19A, there are two sub-pixels 6A and 6B for 2-gradation-level representation in each pixel 3X, in which an area ratio is 1:2. Here, it is assumed that each of the sub-pixels 6A and 6B has two gradation levels. In this case, it could be understood from 2·2=4 that 4-gradation-level representation is possible in the pixel 3X, as shown in FIG. 19B. A numeral 710 indicates a pixel gradation level 0, and both sub-pixels 6A and 6B represent black (for example, a non-lighting condition). A numeral 711 indicates a pixel gradation level 1, the sub-pixel 6B represents black, and the sub-pixel 6A represents white (for example, lighting condition). A numeral 712 indicates a pixel gradation level 2, the sub-pixel 6B represents white, and the sub-pixel 6A represents black. A numeral 713 indicates a pixel gradation level 3, the sub-pixel 6B represents white, and the sub-pixel 6A represents white.
In such a configuration of pixel and gradation, desired gradation characteristics can not be obtained when an image is displayed by using a plurality of pixels which are actually arrayed in a matrix. FIG. 20 shows a gradation from black (shaded portion) to white as an example of image display. This gradation corresponds to change from the pixel gradation level 0 to the pixel gradation level 1, the pixel gradation level 2 and the pixel gradation level 3 shown in FIG. 19B. In FIG. 20, an “arrow ↑ of solid line” indicates the separation between pixels. At the switching from the pixel gradation level 1 to the pixel gradation level 2 in the gradation, a white portion appears for the width of one pixel as shown by “arrows ↑ of dashed lines”. This phenomenon is caused by a movement of a center of the gradation as a whole due to the large difference in gradation representation between sub-pixels of one pixel (difference in brightness between sub-pixels). Hereafter, this phenomenon is referred to as a false contour. Because of such movement of the center of the gradation, this gradation can not be seen as a smooth gradation variation by the human eye, resulting in the deterioration of picture quality. Moreover, in the case of a color gradation, the movement of a center of the color gradation occurs individually for each color, resulting in a problem that a false color appears in a contour portion.
Also, another factor of the deterioration of picture quality is appearance of a periodical pattern associated with periodicity of the sub-pixels as can be seen in the example of the image display in FIG. 20. In FIG. 20, there is a narrow black representation portion in a switching area from the pixel gradation level 2 to the pixel gradation level 3, and the portion appears as a vertical line. Such a pattern is caused by pixel configuration, and can be prevented by a complex pixel configuration (to be more precise, by making a sub-pixel more minute and making the spatial resolution higher so that it can not be felt by the human eye). However, it is not practical to make the pixel configuration more complicated.
In an example of representation shown in FIG. 19A, the number of sub-pixels is 2 and the area ratio is 1:2. The deterioration of picture quality occurs even if the number of sub-pixels is set to n and the area ratio is set to 1:21:22:. . . :2n−1 (n is an integer equal to or more than one) in order to present the further multiple-gradation representation in one pixel.
The present invention is accomplished in view of the above mentioned problems.
Therefore, an object of the present invention is to provide a display apparatus, which uses an area gradation representation method that a pixel is divided into a plurality of sub-pixels, and the deterioration of picture quality due to a pixel configuration effect is suppressed.
Another object of the present invention is to provide a display apparatus, in which picture quality substantially equivalent to that of an analog gradation representation method can be obtained by a combination with a time divisional driving method.
Still another object of the present invention is to provide a display apparatus that can present an area gradation representation with 64-gradation-level representation and high picture quality.
Still another object of the present invention is to provide a display apparatus that can present an area gradation representation with higher picture quality by suppressing difference in brightness.
Still another object of the present invention is to provide a method of gradation representation by which the configuration effect is restrained and higher picture quality is achieved.